1. Field of the Invention
The present invention relates to an optical element, in particular, a highly accurate optical element used for a lens or the like.
2. Description of the Related Art
In recent years, the production of cameras including digital cameras has been increasing, and hence, there have been demanded optical lenses having higher performance. In particular, for enhancing the optical performance of a camera or the like, there is required an optical material having high refractive index and low dispersibility.
The high refractive index and low dispersibility, which are absent in the conventional optical glass, can be realized by using a crystal material, and in order to use the crystal material as a material which has a good transmittance and is suitable for optical applications, there has been a method involving using a single-crystal material or a method involving sintering crystal particles and using the resultant.
On the other hand, there have been problems that the single-crystal material is extremely expensive, and it is difficult to obtain a material which has a large diameter and is suitable for an optical lens. That is, in the method involving sintering crystal particles, when the particle diameter is large, a large grain boundary occurs during sintering to cause decrease in a transmittance as an optical lens, and a defect occurs on a lens surface due to a grain boundary at the time of processing the material into a lens shape, and hence, it has been difficult to obtain a good optical lens.
In Japanese Patent Application Laid-Open No. H06-056514, there is disclosed, as crystal particles each having a small particle diameter, an example of light transmissive ceramics having a crystal particle diameter of 100 nm or less. However, in the process of sintering crystal particles each having a diameter of 100 nm or less, the handling thereof was extremely difficult due to the small bulk density at the time of forming a preliminary molded body before sintering. On the other hand, of the ceramics having optical properties of high refractive index and low dispersibility, there are many substances which have high sintering temperature and are hence accompanied by difficulties during the process of sintering.